The present invention relates to a novel illumination device.
The distribution of light from a source over an area has been a problem since the advent of mankind's effort to eliminate darkness. More specifically, the electrical lamps, incandescent, flourescent, arc discharge and the like, recently developed have been utiltzed in a variety of situations. Recent energy shortages have prompted the development of luminaires to ideally uniformly illuminate an area, especially with a combination of such luminaires. Safety considerations have also prompted a search for a device that illuminates an area susceptible to moving traffic, vehicular or otherwise.
One of the problems which has faced lighting research and development has been the elimination of the intense lumination from , luminaire toward the driver's of a car. Early developments have established the employment of sharp cutoff shields at about 75.degree.. Since every automobile has a different roof line this figure is not precise. Complicating the problem is the fact that the area to be uniformly illuminated requires an uneven luminous intensity away from the luminaire with the maximum intensity concentrated at angles away from the straight down angle (0.degree.). Another aspect of the problem is that light patterns from existing adjacent units will tend to be more uniform when such light patterns overlap, particularly when the spacing of the luminaires increases. This derives from the fact that surface reflectance is greater for light rays at large angles of incidence. Another consideration is that close spacing of the luminaires can provide excellent visibility but this system entails higher costs and more clutter than luminaires distributed in a sparcer pattern.
Although attempts have been made to produce smooth distribution of light patterns with curved reflectors, results have been disappointing. Improvements have been made employing a rectangular light pattern; the square being as the optimum shape for a lighting unit. For example, my U.S. Pat. No. 3,746,854, issued July 17, 1973, l describes a luminaire having special geometric criteria, so-called "tangent light planes." This prior device, was a step forward in luminaire design but offers the disadvantages of marginal heat radiating characteristics because of excessive redirecting of light back to the source and futher loss of projected light as a result of internal lens reflections. Difficulties arise from precise lamp placement, also.
Reference to my article "The Classification Dilemma for Sharp Cutoff Roadway Luminaires", Vol. 65, No. 3, Illumination Engineering, March 1970 recognizes the need for a Maximum Candle Power Angle, thus permitting formation of a maximum candle power beam between maximum candle power planes.
As will be seen from the prior knowledge, the need for a maximum intensity beam at high angles in relation to the straight down axis becomes a greater problem as the maximum candle power angle increases. This, of course, is a result of the illuminance on a surface decreasing inversely as the square of the distance from the source.
No satisfactory luminaire is known that will substantially produce a uniform surface distribution of light at high angles of projection from the source.